The present invention generally relates to a chemical mechanical polisher for polishing semiconductor wafers and a method of using and more particularly, relates to a chemical mechanical polisher that is equipped with a chilled wafer holder and polishing pad and a method for using the chemical mechanical polisher.
Apparatus for polishing thin, flat semiconductor wafers is well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semiconductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad, or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or, similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head; a wafer unload station; or, a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is xe2x80x9cplanarizedxe2x80x9d or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
A schematic of a typical CMP apparatus is shown in FIGS. 1A and 1B. The apparatus 20 for chemical mechanical polishing consists of a rotating wafer holder 14 that holds the wafer 10, the appropriate slurry 24, and a polishing pad 12 which is normally mounted to a rotating table 26 by adhesive means. The polishing pad 12 is applied to the wafer surface 22 at a specific pressure. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing, a metal oxide may be formed and removed repeatedly.
A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off-center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is also rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel. It is known in the art that uniformity in wafer polishing is a function of pressure, velocity and the concentration of chemicals. Edge exclusion is caused, in part, by a non-uniform pressure applied on a wafer. The problem is reduced somewhat through the use of a retaining ring which engages the polishing pad.
Referring now to FIG. 1C, wherein an improved CMP head 20, sometimes referred to as a Titan(copyright) head which differs from conventional CMP heads in two major respects is shown. First, the Titan(copyright) head employs a compliant wafer carrier and second, it utilizes a mechanical linkage (not shown) to constrain tilting of the head, thereby maintaining planarity relative to a polishing pad 12, which in turn allows the head to achieve more uniform flatness of the wafer during polishing. The wafer 10 has one entire face thereof engaged by a flexible membrane 16, which biases the opposite face of the wafer 10 into face-to-face engagement with the polishing pad 12. The polishing head and/or pad 12 are moved relative to each other, in a motion to effect polishing of the wafer 10. The polishing head includes an outer retaining ring 14 surrounding the membrane 16, which also engages the polishing pad 12 and functions to hold the head in a steady, desired position during the polishing process. As shown in FIG. 1C, both the retaining ring 14 and the membrane 16 are urged downwardly toward the polishing pad 12 by a linear force indicated by the numeral 18 which is effected through a pneumatic system.
The enlarged cross-sectional representation of the polishing action which results form a combination of chemical and mechanical effects is shown in FIG. 1B. The CMP method can be used to provide a planner surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An outer layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing, a metal oxide layer can be formed and removed repeatedly.
During a CMP process, a large volume of a slurry composition is dispensed. The slurry composition and the pressure applied between the wafer surface and the polishing pad determine the rate of polishing or material removal from the wafer surface. The chemistry of the slurry composition plays an important role in the polishing rate of the CMP process. For instance, when polishing oxide films, the rate of removal is twice as fast in a slurry that has a pH of 11 than with a slurry that has a pH of 7. The hardness of the polishing particles contained in the slurry composition should be about the same as the hardness of the film to be removed to avoid damaging the film. A slurry composition typically consists of an abrasive component, i.e, hard particles and components that chemically react with the surface of the substrate.
For instance, a typical oxide polishing slurry composition consists of a colloidal suspension of oxide particles with an average size of 30 nm suspended in an alkali solution at a pH larger than 10. A polishing rate of about 120 nm/min can be achieved by using this slurry composition. Other abrasive components such as ceria suspensions may also be used for glass polishing where large amounts of silicon oxide must be removed. Ceria suspensions act as both the mechanical and the chemical agent in the slurry for achieving high polishing rates, i.e, larger than 500 nm/min. While ceria particles in the slurry composition remove silicon oxide at a higher rate than do silica, silica is still preferred because smoother surfaces can be produced. Other abrasive components, such as alumina (Al3O2) may also be used in the slurry composition.
The polishing pad 28 is a consumable item used in a semiconductor wafer fabrication process. Under normal wafer fabrication conditions, the polishing pad is replaced after about 12 hours of usage. Polishing pads may be hard, incompressible pads or soft pads. For oxide polishing, hard and stiffer pads are generally used to achieve planarity. Softer pads are generally used in other polishing processes to achieve improved uniformity and smooth surface. The hard pads and the soft pads may also be combined in an arrangement of stacked pads for customized applications.
Referring now to FIG. 1D, wherein a perspective view of a CMP polishing station 42 is shown. The polishing station 42 consists of a conditioning head 52, a polishing pad 28, and a slurry delivery arm 54 positioned over the polishing pad. The conditioning head 28 is mounted on a conditioning arm 58 which is extended over the top of the polishing pad 28 for making sweeping motions across the entire surface of the pad. The slurry delivery arm 54 is equipped with a single slurry dispensing nozzle 62 which is used for dispensing a slurry solution on the top surface 60 of the polishing pad 56. Surface grooves 64 are further provided in the top surface 60 to facilitate even distribution of the slurry solution and to help entrapping undesirable particles that are generated by coagulated slurry solution or any other foreign particles which have fallen on top of the polishing pad during a polishing process. The surface grooves 64 while serving an important function of distributing the slurry also presents a processing problem when the pad surface 60 gradually worn out after successive use.
In a typical polishing slurry composition of a colloidal suspension of particles, a dispersion agent is added to facilitate the distribution of the particles in the suspension. During a chemical mechanical polishing process, the temperature of the slurry gradually increases by to the mechanical heat generated between the polishing pad, the wafer surface and the slurry particles. The increased temperature of the slurry affects the efficiency of the dispersion agent and as a result, large agglomerates of particles are formed in the slurry suspension. These agglomerates have sizes substantially larger than the surface grooves provided in the polishing pad and thus, cannot be trapped by the surface grooves. The agglomerates of particles can thus become a serious source of contamination leading to significant scratches on the wafer surface. A major scratch on the wafer surface may cause the scrap of the entire wafer.
It is therefore an object of the present invention to provide a chemical mechanical polisher that does not have the scratching defect caused by the formation of large slurry particle agglomerates.
It is another object of the present invention to provide a chemical mechanical polisher that is equipped with a chilled wafer holder and a chilled polishing pad.
It is a further object of the present invention to provide a chemical mechanical polisher wherein any increase in temperature of a slurry solution is controlled.
It is another further object of the present invention to provide a chemical mechanical polishing method without scratching defects caused by the formation of large slurry particle agglomerates.
It is still another object of the present invention to provide a chemical mechanical polishing method by flowing a cooling fluid into a wafer holder and a polishing pad such that the temperature increase of the slurry solution can be controlled.
It is yet another object of the present invention to provide a chemical mechanical polishing method by flowing a high heat capacity fluid into a wafer holder and a polishing pad for controlling the temperature rise of the holder and the pad.
In accordance with the present invention, a chemical mechanical polisher that is equipped with a chilled wafer holder and polishing pad and a method of using the polisher are provided.
In a preferred embodiment, a chemical mechanical polisher that is equipped with a chilled wafer holder and polishing pad is provided which includes a polishing head that has a retaining ring chamber for pressing a retaining ring downwardly, the retaining ring defines a membrane chamber therein for contacting and pressing a wafer downwardly onto a polishing pad; the membrane chamber receives a first heat exchanging fluid for removing heat from the wafer during a polishing process; and a pedestal for mounting a polishing pad on a top surface, the polishing pad has a bottom surface provided with fluid channels for circulating a second heat exchanging fluid therein, the second heat exchanging fluid being fed into the fluid channels through a fluid passageway in the pedestal for removing heat from the polishing pad.
In the chemical mechanical polisher that is equipped with a chilled wafer holder and polishing pad, the membrane chamber may further include an inlet and an outlet for the first heat exchanging fluid, the polisher may include at least one fluid reservoir for holding the first and the second heat exchanging fluid, the polisher may further include a temperature controller for controlling the temperature of the first and second heat exchanging fluid. The first heat exchanging fluid may be water. The polisher may further include a temperature controller for controlling the temperature of the first heat exchanging fluid to not higher than 23xc2x0 C., and preferably to not higher than 18xc2x0 C. The first and the second heat exchanging fluids may be the same, or may be different.
The present invention is further directed to a chemical mechanical polishing method that is without scratching defect caused by large slurry particle agglomerates which can be carried out by the operating steps of first providing a polishing head equipped with a retaining ring chamber and a membrane chamber situated inside the retaining ring chamber; mounting a wafer on the membrane chamber with a surface to be polished exposed; flowing a first heat exchanging fluid into the membrane chamber for pressing the wafer in a downward direction onto a polishing pad and for removing heat during a polishing operation; providing a pedestal for holding a polishing pad thereon; forming a plurality of flow channels on a bottom side of the polishing pad for intimately contacting a top surface of the pedestal; and flowing a second heat exchanging fluid into the plurality of flow channels and removing heat from the polishing pad during a polishing process such that the generation of large slurry particle agglomerates is avoided.
The chemical mechanical polishing method that is without scratching defect caused by large slurry particle agglomerates may further include the step of providing fluid passageways in the pedestal for flowing the second heat exchanging fluid into the plurality of flow channels. The method may further include the step of selecting the first heat exchanging fluid from one that has a heat capacity of at least that of water. The method may further include the step of flowing the first heat exchanging fluid of water at a temperature not higher than 23xc2x0 C. into the membrane chamber, or flowing the first heat exchanging fluid of water at a temperature preferably not higher than 18xc2x0 C. into the membrane chamber. The method may further include the step of selecting the second heat exchanging fluid the same as the first heat exchanging fluid, or selecting the second heat exchanging fluid different than the first heat exchanging fluid. The method may further include the step of forming the plurality of flow channels on the bottom side of the polishing pad to a depth that is not larger than xc2xe of the thickness of the pad. The method may further include the step of providing a fluid inlet and a fluid outlet on the membrane chamber.